Many products have specific or preferred container closure requirements and methods. In particular, products such as wines have strict requirements for container closures. Wine generally is sold in vertically-oriented bottles with a narrow circular opening at the top of the container. There are many requirements placed upon the closure systems for wine sold in bottles because of the delicate nature of the product. Due to the strenuous requirements for closure systems in wine bottles, most wine bottle closures traditionally have been produced from natural cork.
The use of natural cork container closures dates back to the 17th century. A natural cork closure is produced from the outer bark of the cork oak species “Quercus Suber,” a tree that is predominant around the Mediterranean Sea. Natural cork container closures have favorable properties attributed to a high density closed-cell structure, i.e., more than 20 million cells per cm3, and a very thin cell wall, i.e., 1 to 2 microns. Natural cork also has excellent mechanical properties, namely compressibility and elasticity, which have made natural cork a material of choice for production of closures for wine glass bottles.
However, natural cork is not without limitations. For example, cork is available only in a specific geographic area and quantities are limited, causing prices to escalate. Furthermore, since natural cork is a natural product and subject to non-controllable climate conditions during the growth of the tree, cork shows a relatively high variation in properties, even within different quality sub-groups, namely, regarding its “oxygen transmission rate” (OTR). Additionally, natural cork is prone to develop a “cork taint” (rotten cardboard) smell and flavor linked to the presence of minute contamination with TCA (2,4,6-tricloranisole), which is believed to affect up to around 5% of all wine bottles.
These limitations promoted the development of “alternative container closures,” which seek to overcome these limitations and/or emulate the best properties of cork.
Starting in the 1950s, “technical corks” were developed. Technical corks included “colmated” corks (filling in the surface voids of a lower quality natural cork with cork powder and a binder to improve surface homogeneity and reduce permeability), “agglomerated” corks (cork particles compressed together with a binder), 1+1 corks (an agglomerated cork with two cork discs, one on each side), etc. The technical corks were based on natural cork, but included additional manipulation to overcome some of the above limitations.
The early 1990s saw a technological discontinuity with the introduction of synthetic “plastic” container closures that used the same geometric configuration of the cork container closure, the same sealing mechanism for the same type of container (glass bottle), the same application (jaw clamping), and the same removal (cork screw) equipment. Later introductions included the “screw-cap”, already extensively used for other beverages and also for sweet wines and liqueurs, with sealing done on the outside of the bottle neck. Outside sealing required different bottles and different bottling equipment.
These new container closures have consistently gained market share, initially among the “New World” wine producers and in young, fruity, white wines to be drunk within a year or two after bottling. New container closures have subsequently expanded the market to include more “long-term” types of wines.
In the eyes of many informed consumers, a cork container closure is still associated with a premium choice for high-quality wines (reds in particular) that are to be kept for a number of years. This segment is still largely untapped by synthetic container closures or screw caps.
Synthetic foamed plastic wine container closures have tried to overcome some of the above-mentioned limitations of the traditional cork closure.
In terms of the production processes, there are several main synthetic container closure families of products:                a. Injected container closures—obtained by a batch “injection-molding” process. These container closures are easily recognizable by a top and bottom surface finish that matches the finish on the cylinder side wall;        b. Extruded container closures—continuously extruded through an extruder die and length slit as required. These container closures usually have a more homogeneous structure and small cell size often visible on the top and bottom ends; and        c. Bead molded container closures—which are made by fusing foams beads together in a mold. These container closures have the advantage of retaining the appearance of natural cork. These closures have good compressive resistance because of a uniform cellular structure.        
The overall performance of these synthetic container closures has progressively improved over the years, with the use of new thermoplastic materials, more elaborate compositions (density, two-layer sequential extrusion, etc.), and better control over the production process.
On the positive side, synthetic container closures are much more consistent than natural products, so synthetic closures generally exhibit lower standard deviation between different samples than same-quality natural cork container closures.
Obviously, all raw materials, such as polyolefins, block copolymers, ethylene copolymers, etc., are preferably organoleptic neutral, i.e., no taste or smell conferred upon or removed/scalped from the wine.
Some mechanical properties of a container closure are easily measured: compressibility, which affects insertion both on the usual bottling lines and for hand re-insertion after opening; expansion rate after insertion, which affects the immediate sealing properties and the manipulation time-lag after bottling; and relaxation force, which must be sufficiently large enough to guarantee a good seal but low enough for cork-screw removal.
Generally speaking, it is fair to say that, for the type of foamed materials currently being used for container closures, the higher the density the higher the “stiffness”, i.e., a high density material is less compressible (or requires a larger compression force to achieve the same deformation), exhibits a higher relaxation force, and also requires a higher removal force. A “soft” low density container closure will be easier to insert and to remove but will show a smaller relaxation force (i.e., it will take longer to properly seal the bottle) and will probably show permanent deformation in the future. Thus, even based upon these purely mechanical performance evaluation criteria, there is a need to find an acceptable compromise between force (ease) of insertion and removal and good, fast, and permanent sealing properties.
A main limitation of plastic synthetic container closures is a relatively high oxygen transmission rate (OTR) through the closure and/or through the glass-closure interface. Different container closure materials and container closure designs show different OTRs; but it is now clear that, in general, synthetic container closures show a higher OTR than the best quality cork container closures or screw caps. This is the main reason why synthetic container closures have primarily captured the “young” wine market segment and have failed to obtain similar gains in wines kept for several years before opening.
For a foamed “plastic” container closure, the closure OTR is closely lined to the cell structure and density, with the actual closure material having only a limited influence on the final value. Different plastic materials have slightly different OTRs, but the density and the cell sizes primarily affect OTR.
Here again, for these foamed materials, a good OTR performance (low transmission rate) is substantially achieved with high density, i.e., hard, stiff container closures. Difficulties exist with creation of a low OTR plastic closure that also shows acceptable mechanical properties.
Choosing a container closure is no longer merely a way of sealing the wine in the bottle at the lowest possible cost. The container closure and the storage conditions determine the wine evolution in the bottle. A rewarding wine tasting experience after a certain period of time only materializes if all components that affect the wine evolution are coherently matched to the required and expected outcome.
Different container closures with different OTRs have a profound impact on the way a given wine evolves and develops over the years inside the bottle. Current theories in enology suggest that the role of an enologist does not end when wine is bottled, but extends until wine is served to a consumer. The kind of sensorial evolution that the wine goes through in the container is a major factor conditioning the consumer experience. Therefore, bottling conditions and all factors, including the container closure, affecting the sensorial evolution should be under direct control of the enologist. The selection and specific properties of the container closure preferably is a major responsibility of the enologist and may affect how the wine tastes when the container is opened. The choice of container closure has become critical to the future consumer experience. The choice of closure controls the slow oxidation, reduction, or polymerization reactions that a bottled wine goes through inside the bottle (all other conditions being equal—bottle size, temperature, temperature cycles, vertical or horizontal bottle keeping, etc.).
A “universal” container closure that equally suits all types of wine and all storage times is not practical. Different wines and different storage times need different closures with different OTRs, but all must show similar mechanical properties. The wine technologist/enologist must determine what OTR properties are required so an individual wine reaches optimum maturity levels after a specified number of years of storage under ideal conditions.
With such new requirements, several producers of synthetic container closures are starting to bring on the market different closures with different OTRs and trying to extend the suggested wine storage period. A lack of new advances in the field is limiting progress.
Another problem with developing container closures for the wine industry is the need for closures to withstand substantial pressure buildups that occur during storage of the wine product after bottling and sealing. During natural expansion of wine during hotter months, the pressure within bottles increases and imposes a burden upon the closure that must be resisted. Displacement of the closure out of the bottle must be prevented. As a result, a container closure must be capable of secure, intimate, frictional engagement with the bottle neck in order to resist any such pressure build ups.
In the wine industry, a secure sealed engagement of the closure with the neck of the bottle must be achieved virtually immediately after the closure is inserted into the neck of the bottle. During normal wine processing, the container closure is compressed, as detailed above, and inserted into the neck of the bottle to enable the closure to expand in place and seal the bottle. However, such expansion should occur immediately upon insertion into the bottle, since many processors tip the bottle onto its side or neck down after the closure is inserted into the bottle neck allowing the bottle to remain stored in this position for extended periods of time. If the closure is unable to rapidly expand into secure, intimate, frictional contact and engagement with the walls of the neck of the bottle, leakage will occur.
Container closures preferably are removed from a bottle using a reasonable extraction force. Although actual extraction forces extend over a wide range, generally accepted, conventional extraction forces are typically below 100 pounds. A balance must be achieved between secure sealing of a bottle and providing a reasonable extraction force for removal of the closure from the bottle. Since the requirements for these two characteristics are in direct opposition to each other, a careful balance must be achieved so that the closure is capable of securely sealing the wine in the bottle, preventing both leakage and gas transmission, while also being removable from the bottle without requiring an excessive extraction force.
Existing alternative systems are not adequate to satisfy the demanding requirements of the wine bottling industry. Thus, a need exists for improved synthetic closures for containers with improved closure properties. A new approach to the design of plastic closures is required.